The use of electronics in measuring performance of IC engines is well known. One such performance measurement is relative compression which provides an indication of the engine's balance between cylinders. The prior art methods for electronically measuring relative compression in vehicle mounted engines involve cranking the engine without ignition and measuring the change in engine sub-cyclic speed due to each cylinder's compression stroke. Two such prior methods are disclosed in commonly assigned U.S. Pats. Nos. 4,050,296 to Benedict, entitled RELATIVE COMPRESSION OF AN INTERNAL COMBUSTION ENGINE, and 4,348,893 to Hendrix et al, entitled RELATIVE COMPRESSION OF AN ASYMMETRIC INTERNAL COMBUSTION ENGINE.
In Benedict the engine is cranked without ignition and the peak to peak excursion in starter current occuring over each cylinder sub-cycle is measured and compared with an average peak to peak magnitude obtained for all cylinders. The peak to value of interest covers the cylinder compression and expansion strokes.
In the patent to Hendrix et al an asymmetric IC engine is similiarly cranked without ignition and the peak to peak change in sub-cyclic crankshaft speed is measured, rather than starter current. Speed is measured by sensing the rate of rotation of the engine's ring gear teeth past a fixed point. Hendrix et al alternates between measuring the maximum to minimum and minimum to maximum changes in crankshaft speed so as to measure the compression stroke for left bank cylinders and the expansion stroke for the right bank cylinders.
Each of these methods are limited in both implementation and accuracy. Since each involve measurement of engine compression without ignition, neither may be performed during normal vehicle operation. Instead the tests are relegated to garage procedures, i.e. maintenance tests performed in a repair facility. Similiarly, compression measurement without engine operation means that the compression data is not taken with respect to optimum engine conditions. It is taken at cranking rather than idle speeds. This further results in poor correlation of test data since other engine performance characteristics may dominate, such as faulty starter problems.